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Borrowing Limits, Amounts Outstanding and Amounts Available Under Revolving Credit Facility (Detail) (Revolving Facility, USD $)
In Millions, unless otherwise specified
Sep. 30, 2012
Dec. 31, 2011
Sep. 30, 2011
Revolving Facility
     
Debt Instrument [Line Items]      
Revolving credit facility $ 60.0 $ 115.0 $ 80.0
Outstanding revolving credit facility 24.1 87.9 65.4
Available revolving credit facility $ 35.9 $ 27.1 $ 14.6